LM3488MM datasheet(21/24 Pages) NSC

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Designing SEPIC Using LM3488

(Continued)

Peak current in the inductor, to ensure the inductor does not

saturate:

L1PK must be lower than the maximum current rating set by

the current sense resistor.

The value of L1 can be increased above the minimum rec-

ommended to reduce input ripple and output ripple. How-

ever, once D

IL1 is less than 20% of IL1AVE, the benefit to

output ripple is minimal.

By increasing the value of L2 above the minimum recom-

mended,

∆

IL2 can be reduced, which in turn will reduce the

output ripple voltage:

where ESR is the effective series resistance of the output

capacitor.

If L1 and L2 are wound on the same core, then L1 = L2 = L.

All the equations above will hold true if the inductance is

replaced by 2L. A good choice for transformer with equal

turns is Coiltronics CTX series Octopack.

SENSE RESISTOR SELECTION

The peak current through the switch, I

SW(PEAK) can be ad-

justed using the current sense resistor, R

SEN, to provide a

certain output current. Resistor R

SEN can be selected using

the formula:

Sepic Capacitor Selection

The selection of SEPIC capacitor, CS, depends on the rms

current. The rms current of the SEPIC capacitor is given by:

The SEPIC capacitor must be rated for a large ACrms cur-

rent relative to the output power. This property makes the

SEPIC much better suited to lower power applications where

the rms current through the capacitor is relatively small

(relative to capacitor technology). The voltage rating of the

SEPIC capacitor must be greater than the maximum input

voltage. Tantalum capacitors are the best choice for SMT,

having high rms current ratings relative to size. Ceramic

capacitors could be used, but the low C values will tend to

cause larger changes in voltage across the capacitor due to

the large currents. High C value ceramics are expensive.

Electrolytics work well for through hole applications where

the size required to meet the rms current rating can be

accommodated. There is an energy balance between CS

and L1, which can be used to determine the value of the

capacitor. The basic energy balance equation is:

Where

is the ripple voltage across the SEPIC capacitor, and

is the ripple current through the inductor L1. The energy

balance equation can be solved to provide a minimum value

for C

Input Capacitor Selection

Similar to a boost converter, the SEPIC has an inductor at

the input. Hence, the input current waveform is continuous

and triangular. The inductor ensures that the input capacitor

sees fairly low ripple currents. However, as the input capaci-

tor gets smaller, the input ripple goes up. The rms current in

the input capacitor is given by:

The input capacitor should be capable of handling the rms

current. Although the input capacitor is not as critical in a

boost application, low values can cause impedance interac-

tions. Therefore a good quality capacitor should be chosen

in the range of 100µF to 200µF. If a value lower than 100µF

is used, then problems with impedance interactions or

switching noise can affect the LM3478. To improve perfor-

mance, especially with V

IN below 8 volts, it is recommended

to use a 20

Ω resistor at the input to provide a RC filter. The

resistor is placed in series with the V

IN pin with only a bypass

capacitor attached to the V

IN pin directly (see Figure 13). A

0.1µF or 1µF ceramic capacitor is necessary in this configu-

ration. The bulk input capacitor and inductor will connect on

the other side of the resistor with the input power supply.

Output Capacitor Selection

The ESR and ESL of the output capacitor directly control the

output ripple. Use low capacitors with low ESR and ESL at

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